The present invention relates to the optical fiber telecommunication networks, comprising Synchronous Optical NETwork (SONET) rings or linear SONET systems, and multiple Wavelength Division Multiplexing (WDM) point-to-point links, and more particularly, to methods and system for communication between optical and SONET layers in mixed WDM/SONET networks.
WDM link capability to upgrade the network capacity without using additional fiber has brought about wide-scale deployment of WDM systems carrying many independent traffic channels over the same fiber. Deployment of WDM links shows that an optical layer having its own functions has emerged as a new independent layer in telecommunication networks. The optical layer requires independent service protection (restoration) in case of fiber or cable cuts. Methods of service protection for mixed WDM/SONET networks were suggested by the inventor of the subject patent application in the U.S. patent application Ser. No 08/867,346 "Method and System for Service Restoration in Optical Fiber Communication Networks" and the US Patent Application "System and Method for Protection of WDM/SONET Networks" filed Dec. 4, 1997. Optical switching technology was utilized to switch WDM traffic from a failed WDM link to a dedicated protection ring (DPR). The dedicated protection ring was selected from a plurality of SONET rings passing through the WDM link. Optical switches deployed at transmitting and receiving WDM terminals of DPR nodes rerouted WDM traffic from working fibers of WDM link to protection fibers of DPR.
Beyond adequate network architecture assuming the WDM traffic rerouting in case of cable cuts, a system and method had to be developed to detect possible failure, transfer the required information along DPR, control optical switch operation and report alarms. In currently deployed mixed WDM/SONET networks, communication between the network nodes is conducted independently on optical and SONET layers. WDM systems have their own service channels, usually one of out-of-band WDM channels dedicated for transport of device status and alarms. SONET terminals and add/drop multiplexers (ADMs) communicate with each other using SONET overhead. Overhead consists of several components such as section, line and path, each carrying certain functions [B. Kumar, Broadband Communications: a Professional Guide to ATM, Frame Relay, SMDS, SONET, and BISDN, McGraw-Hill, NY, 1995, pp. 164-181]. The overhead bytes responsible for Automatic Protection Switching (APS) are included in line overhead. Detailed description of APS functionality may be found, for example, in a book of T. H. Wu, "Fiber Network Service Survivability", Norwood, Mass., Artech House, 1992.
Optical switching technology has not been commercially deployed yet, and only experimental networks and testbeds were investigated. For demonstration of optical switching functionality, non-standard signaling protocols were used in these experimental networks and testbeds. However these non-standard protocols are not sufficient to detect failures in the optical domain, control optical switches and interact with SONET protocol. In present mixed WDM/SONET networks comprising WDM links and SONET rings, no interaction exists between SONET and optical layers. Introduction of optical switching technology into mixed network environment brought a necessity of communication between SONET and WDM control protocols. In principle, this communication may be introduced in Operation, Administration, Management and Provisioning (OAM&P) software. For WDM systems with many channels, OAM&P software upgrade becomes a global problem, because each SONET ring has to be managed separately. Also, time required for OAM&P response may exceed the maximum time interval tolerated by SONET standard for protection switching (about 50 msec). In the existing networks with SONET and WDM equipment, this software upgrade is difficult or impossible.
It is therefore desirable to develop a systematic approach to optical layer control for providing network survivability and compatibility with SONET protection mechanisms and other SONET functions through communication between optical and SONET layers.